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The Structure and Function of Carbohydrates

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A carbohydrate is an organic molecule containing only Carbon, Hydrogen and Oxygen with the general formula Cn(H20)n. They are made up of individual molecules called monomers which are joined together by condensation reactions to make a longer chain called a polymer. Carbohydrates are categorised in to three main groups; monosaccharides, disaccharides and polysaccharides.

Monosaccharides are single sugars and are sweet tasting soluble substances such as glucose and fructose which are the building blocks for all carbohydrates. They are used as energy in respiration by being broken down in to carbon dioxide and water. ATP, an immediate energy source, is released during the glycolysis and the Krebs cycle whilst also being generated through the electron transfer chain. Monosaccharides are classified according to the number of carbon atoms, if it contains 3 carbons they are called Trioses e.g. glyceraldehydes which plays a part in respiration and photosynthesis. Pentoses e.g. ribose, contain 5 carbons and are used in the synthesis of nucleic acids (RNA and DNA), providing a strong sugar-phosphate backbone. When they join together a disaccharide is formed. The reaction in which this takes place is called a condensation reaction and it involves the loss of water (H2O) and the formation of a glycosidic bond. The reverse of this reaction, the formation of two monosaccharides from one disaccharide, is called a hydrolysis reaction and requires one water molecule to supply the Hydrogen and Oxygen-Hydrogen to the sugars formed.

Some examples of disaccharides include Sucrose (glucose + fructose) which is used in many plants for transporting food reserves, often from the leaves to other parts of the plant, Lactose (glucose + galactose) which is the sugar found in the milk of mammals and Maltose (glucose + glucose) which is the first product of starch digestion and is further broken down to glucose before absorption in the human gut. All monosaccharides and some disaccharides including maltose and lactose are reducing sugars. Their presence can be tested by adding Benedict’s reagent to the sugar and heating in a water bath. If a reducing sugar is present, the solution turns green, then yellow and finally produces a brick red precipitate. Non-reducing sugars can also be tested for using Benedict’s reagent but first requires the addition of an acid and heating to hydrolyse the sugar. The acid must then be neutralised using an alkali such as sodium hydroxide before carrying out the test as described above. Polysaccharides are complex carbohydrates formed from many monosaccharides. A number of monosaccharide molecules such as glucose become linked by glycosidic bonds with the elimination of a molecule of water for each monosaccharide added in condensation reactions to form long chains.

Through hydrolysis, these polymers are broken down again in to disaccharides and monosaccharides with the addition of water. An example of a polysaccharide is starch which is the main storage of energy in plants and is insoluble therefore good for storage. It is made up of two polymers, amylose and amylopectin. Amylose is a polymer of glucoses which forms a helix structure which is very compact. Amylopectin is also a polymer of glucoses but with a branched structure rather than helical which mean the compound can easily hydrolyse to release the glucose monomers. Iodine solution or potassium iodide solution can be used to test the presence of starch. A positive result changes the solution from an orange-brown to a blue-black colour. A second example of a polysaccharide is Glycogen which is the main storage of energy in animals and fungi. It is similar to amylopectin but has many more branches which are shorter.

This means that it is extremely compact and hydrolyses very quickly. A final example of a polysaccharide is cellulose which is the main constituent of plant cell walls. It has adjacent chains of long, unbranched polymers of glucose which bond together to form micro fibrils. These layers are orientated in different directions and interwoven providing rigid cell wall with gaps in layers to provide permeability. The carbohydrate glucose is formed in the light-independent reaction which occurs in the stroma. Carbon dioxide combines with RuBP to form a highly unstable 6-carbon compound which splits in to two glycerate-3-phosphates which is then reduced to form two trios phosphate. This can either be recycled back to RuBP or go on to synthesise glucose. In conclusion, carbohydrates are essential for the storage of energy, structural support for example as the backbone of DNA and are an intermediate in respiration e.g. glyceraldehydes.

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